Not Applicable.
Not Applicable.
This invention relates generally to electrostatic precipitators for air pollution control and, more specifically, concerns the control of the rapping process used to clean the internal collection plates and discharge electrodes of electrostatic precipitators.
Continuous emphasis on environmental quality has resulted in increasingly strenuous regulatory controls on industrial emissions. One technique which has proven highly effective in controlling air pollution has been the removal of undesirable particulate matter from a gas stream by electrostatic precipitation. An electrostatic precipitator is an air pollution control device designed to electrically charge and collect particulates generated from industrial processes such as those occurring in cement plants, pulp and paper mills and utilities. Particulate is negatively charged. These negatively charged particles are attracted to, and collected by, positively charged metal plates. The cleaned process gas may then be further processed or safely discharged to the atmosphere.
During continuous operation of an electrostatic precipitator, the collector plates, electrodes and other precipitator internal components must be periodically cleaned to remove the dust build-up which accumulates on these surfaces. The cleaning mechanism typically consists of a mechanical rapper. An electronic rapper controller determines the sequence, intensity, and duration of rapping. Once the particulate is dislodged from the plates, it falls into collection hoppers at the bottom of the precipitator.
Rappers are electro-mechanical devices that are used to mechanically dislodge collected particulate/materials within an electrostatic precipitator, electronic filter or dust collector (hereafter referred to as ESP) by applying direct current (DC) energization to the rapper. In general, a rapper consists of a hammer that mechanically strikes an anvil. The anvil is mechanically connected to the internal components of the ESP, such as the discharge electrodes, collecting plates, gas distribution devices or any other component cleaned by the rapper. Striking the rapper shaft or anvil with the hammer transmits mechanical forces to these components to dislodge collected materials. Several rapper variations exist which may be employed in the cleaning process.
One rapper variation consists of a cylindrical hammer or plunger and solenoid coil. The hammer rests on the rapper shaft or anvil. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force overcomes the force of gravity and lifts the hammer vertically to a height that is determined by the amplitude and length of time of the energization. When the energization is terminated, the electromagnetic field is removed and the hammer drops due to gravitational forces and strikes the anvil. The hammer then rests on the anvil until the next energization.
Another rapper variation places a spring behind the cylindrical hammer. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force will overcome the force of gravity and lift the hammer vertically compressing the spring against the rapper assembly. The height and spring compression are determined by the amplitude and length of time of the energization. When the energization is terminated the hammer strikes the anvil with a force that is comprised of gravitational force plus the spring expansion.
Another rapper variation places a spring behind the cylindrical hammer. This spring is connected to the hammer and holds it above the anvil. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force will overcome the force of the spring and accelerate the hammer downward to strike the anvil. When the energization is terminated, the hammer is returned to position by the spring.
In practice, numerous operational problems associated with the cleaning process may be experienced. Excessive rapping results in the particulate billowing from the plate into the gas stream where it is re-entrained in gas flow and must be recaptured. Otherwise, the re-entrained dust will be discharged from the exhaust stack, resulting in unacceptable emissions into the atmosphere. Insufficient rapping prevents the particulate from falling from the surfaces to be cleaned. In either case, collection efficiency of the precipitator is reduced which reduces the gas volumes that can be treated by the precipitator. In most industrial applications there is a direct correlation between precipitator capacity and production capacity. Therefore, there are significant monetary benefits to be derived from maximizing rapper efficiency. Also, grossly inefficient precipitators which allow an excessive amount of particulate emissions into the atmosphere can prompt the Environmental Protection Agency to shut a particular process down indefinitely.
Rapper control has been typically limited to manually controlling and adjusting the current level to an entire group of rappers, rather than individual rapper control. However, rappers in different locations within the group may operate more efficiently with different current levels. Because the number of rapper groups, as well as the number of rappers within each group, may vary and prior art rapper control only allows for intensity adjustment of an entire group, a compromise in control standards therefore prevails. The result is often rapper inefficiencies that reduce precipitator and production capacity as well as increase emissions levels.
Newer rapper control systems that allow for adjusting individual rappers also experience inefficiencies. These systems fail to take into account line losses and impedance at the rapper coil when making adjustments to the peak current level of an individual rapper. Using an incorrect peak current results in rapper inefficiencies that include reduced precipitator functionality and increased emission level.
Additionally, and important to the present invention, the intensity of the rap and the corresponding cleaning forces imparted to the internal components of the ESP are determined by the height of the hammer is lifted. This is known as the rapper lift. If the hammer is not lifted high enough, then there will be insufficient cleaning. Conversely, if the hammer is lifted too high, then damage to the internal components of the ESP will result. Therefore, it is desirable to closely regulate rapper lift to provide thorough cleaning without damage. It is an object of this invention to provide a system to closely and accurately regulate the rapper lift.
It is well known in the prior art that rapper lift can be measured by a number of conventional methods. For example, a Linear Variable Displacement Transducer (LVDT) can be used to accurately measure rapper lift. U.S. Pat. No. 5,114,442, Artz, teaches that a kinetic energy sensor can be used to measure rapper energy output. Each of these methods has a disadvantage; they require significantly more equipment and wiring than normally found on the conventional rapper control. In some cases, the sensors and wiring may cost more than the rappers, making the application of these methods impractical. U.S. Pat. No. 5,173,867, Johnston et al., discloses a multiple rapper control which has a current detecting means bi-directionally connected to a computer to sense and measure the peak electrical input current to each rapper. It is an object of this invention to provide a system to closely and accurately regulate rapper lift using such a multiple rapper control without requiring additional wiring or sensors.
It is well known in the prior art that the rapper lift is not consistent for a given rapper energization. U.S. Pat. No. 5,114,442, Artz, teaches that an ambient temperature change will cause a change in rapper lift. Artz discloses a system to compensate for temperature changes by adjusting the rapper energization in response to a temperature sensor. This system has a number of disadvantages, not the least of which is the use of temperature sensors or a plurality of temperature sensors to infer a change in rapper characteristics. It is an object of this invention to directly use the individual rapper characteristics to closely and accurately regulate the rapper lift without additional sensors or wiring.
It is well known in the prior art that the rapper lift is not consistent for a given rapper placement on the ESP relative to the rapper control. Rappers may be physically close to the rapper control or some distance away. This has the effect of inserting the impedance of the connecting wire and connections in series with the rapper coil. Although the magnitude of this impedance is not great, it has a substantial effect on the resultant rapper lift. The effect of rapper placement and wiring length on rapper lift was tested using a rapper of the type manufactured by BHA Group Holdings, Inc. of Kansas City, Mo. The rapper is a 120VDC electromagnetic gravity impact style, with a 20-pound hammer. The results are shown in Table 1.
In order to obtain a baseline for comparison, the rapper was tested with an intensity or duration setting of 14.0 half cycles. The rapper was physically close to the rapper control which provided an impedance of zero ohms between the rapper control and the rapper""s coil. This yielded a baseline for comparison of 5.76 inches of rapper lift as shown in Table 1. Keeping the same intensity or duration setting and physically moving the rapper away from the rapper control yielded the remaining results shown in Table 1. Note that the distance is length of wiring, not distance from the rapper control, because it is the length of wiring that adds impedance. The error in rapper lift becomes large with small additions of impedance. It is an object of this invention to provide a system, which will closely and accurately regulate rapper lift independent of wiring impedance or rapper placement on the ESP.
It is well known in the prior art that the rapper lift is affected by the magnitude of the rapper current. U.S. Pat. No. 4,285,024, Andrews, teaches that hammer displacement is indicated by integrating the rapper current over the time the rapper energization pulse is applied. Since accurately measuring rapper lift is fundamental to accomplishing the objectives of this invention, this technique was tested using a rapper of the type manufactured by BHA Group Holdings, Inc. of Kansas City, Mo. The rapper is a 120VDC electromagnetic gravity impact style, with a 20-pound hammer. The results are shown in Table 2.
In order to obtain a baseline for comparison, the rapper was tested with a rapper lift of 5.76 inches. The rapper was physically close to the rapper control, thus providing an impedance of zero ohms between the rapper control and the rapper""s coil. The current was integrated over time, which yielded a baseline for comparison of 1.64 amp seconds. Keeping the same rapper lift and physically moving the rapper away from the rapper control, yielded the results shown in Table 2. Again, the distance is length of wiring, not distance from the rapper control, because it is the length of wiring that adds impedance. If there had been no error, there would have been a reading of 1.64 amp seconds for every case. The error in rapper lift measurement by integrating the current over time is significant and this method will not provide a.measurement accurate enough to accomplish the objectives of this invention. It is therefore an object of this invention to provide a system to more accurately measure rapper lift than prior art systems that can be calibrated in conventional units such as inches or centimeters.
Generally described, a method for controlling electrostatic precipitators and, particularly, for establishing control parameters in electrostatic precipitators, is provided. In accordance with the method, preferably embodied within a computer system, data indicative of performance characteristics of a first rapper is obtained and a first rapper lift value is calculated for the first rapper. Further, the present invention obtains data indicative of performance characteristics of a second rapper and calculates a second rapper lift value for the second rapper. The first rapper lift value is compared with the second rapper lift value and the performance characteristic data of the second rapper is adjusted so that the second rapper lift value approximately equals the first rapper lift value. This process is repeated for each rapper in the system so that each rapper has performance characteristics approximate the characteristics of the first rapper.
The present invention provides an improved method for controlling the rapping process used to clean the internal collection plates and discharge electrodes of electrostatic precipitators. The system obtains the performance characteristics of a first rapper and calculates a rapper lift value. The system then obtains performance characteristic data from two or more additional rappers (preferably each additional rapper in the system) and calculates rapper lift values for each of the rappers. The performance characteristics of each rapper are determined from data indicative of the peak current, current duration and rapper lift for each rapper. Finally, the system compares the rapper lift value of each additional rapper with the determined lift of the first rapper and adjusts the performance characteristics of each of the two or more additional rappers so that their rapper lift values are approximately equal to the performance characteristics of the first rapper and, in particular, so that the performance characteristics of all rappers are approximately equal to one another.